The answer lies in how you define sales: It appears that the Tesla Model S electric sport sedan is outselling both the BMW 7-Series and the Mercedes-Benz S Class in what's called the "large luxury" segment of the U.S. market.

It also apparently outsells the Audi A8, the Lexus LS, and a handful of other full-size sedans from luxury or near-luxury makes.

In comparison, here are the first-quarter sales for the large luxury competitors:

Audi A8:1,462

BMW 7-Series: 2,338

Lexus LS: 2,860

Mercedes-Benz S Class: 3,077

There is one car in the large luxury segment that outsold the Tesla Model S, however.

That's the 2013 Cadillac XTS, the newly introduced replacement for the previous DTS (itself derived from the old De Ville). Fully 7,130 XTSes were delivered from January through March.

However, we'd agree with commentators who suggest that, despite its strides in design, features, and technology, the Cadillac brand still doesn't compete head-to-head with the German luxury incumbents.

And we strongly doubt many Tesla Model S buyers considered an XTS as an alternative--or even know where their local Cadillac dealer is.

It's important to note, however, that this doesn't mean Tesla sold more cars than these brands.

It competes in only a single segment of the market, with one vehicle class.

The established luxury brands, on the other hand, offer many models in many segments--from compact sedans and crossovers to "sports activity coupes" and "four-door coupes" and a host of other types of vehicle, both standard and not-so-standard.

Tesla has said it might deliver 20,000 to 25,000 cars this year--not all of them in the U.S. For comparison purposes, here are the 2012 U.S. sales totals for the top luxury brands: